In a typical cellular wireless communication system, an area is divided geographically into a number of cells and cell sectors, each defined by a radio frequency (RF) radiation pattern from a respective base transceiver station (BTS) antenna. The base station antennae in the cells are in turn coupled with a base station controller (BSC), which is then coupled with a switch or gateway that provides connectivity with a transport network such as the public switched telephone network (PSTN) or the Internet. For instance, the BSC may be coupled with a mobile switching center (MSC) that provides connectivity with the PSTN and/or the BSC may be coupled with a packet gateway, such as a packet data serving node (PDSN) or media gateway (MG), that provides connectivity with the Internet.
When a mobile station, such as a cellular telephone, pager, or wirelessly-equipped computer, is positioned in a cell, the mobile station communicates via an RF air interface with the BTS antennae of a cell. Consequently, a communication path can be established between the mobile station and the transport network, via the air interface, the BTS, the BSC and the switch or gateway.
Further, in most wireless communication systems, multiple BTSs are connected with a common BSC, and multiple BSCs are connected with a common switch or gateway. Each BSC may then manage air interface resources for multiple wireless coverage areas (e.g., multiple cells and sectors), by performing functions such as assigning air interface traffic channels for use by mobile stations in the coverage areas and orchestrating handoff of calls between coverage areas. And the switch and/or gateway, in turn, may control one or more BSCs and generally control wireless communications, by performing functions such receiving and processing call requests, instructing BSCs when to assign traffic channels, paging WCDs, and managing handoff of calls between BSCs.
Unlike landline telephones that exist at known, fixed locations, mobile stations can operate at virtually any location where a wireless carrier provides coverage. Consequently, in order for a mobile station to be able to engage in useful communications (e.g., voice or data) in the cellular wireless communication system, the mobile station must first register with the system, so that the system knows where the mobile station is located (e.g., for purposes of directing calls to the mobile station) and so that the system can verify that the mobile station is authorized to be operating in the system, or for other reasons.
The manner in which a mobile station registers with a cellular wireless communication system can take various forms, depending on factors such as the configuration of the system and on the communication protocols used. In a CDMA system, for instance, a mobile station registers by sending over the air to the base station an “access probe,” which carries an identifier of the mobile station and perhaps other pertinent information. The mobile station sends the access probe in a “slotted aloha process” in which it repeatedly sends the access probe at increasingly higher power levels until it receives an acknowledgement message from the base station, or until it otherwise exhausts the process (e.g., the maximum transmission power of the mobile station is reached and no acknowledgment has been received). The mobile station may repeat a slotted aloha sequence a number of times, until concluding that an access failure has occurred.
Under CDMA, the air interface between the base station and mobile stations is divided into a number of specially coded channels through which this access probe communication occurs. In particular, on the reverse link (extending from the mobile stations to the base station), the air interface defines at least one time-slotted “access channel,” and on the forward link (extending from the base station to the mobile stations), the air interface defines at least one time-slotted “paging channel.” Each access probe travels in a timeslot of an access channel to the base station, and each acknowledgement travels in a timeslot of a paging channel from the base station.
In a sector where multiple paging channels are provided, each mobile station operating in the sector may select a paging channel to use (i.e., to listen to) by applying a hash function keyed to the mobile station's electronic serial number. Similarly, if multiple access channels are provided, each mobile station may select an access channel to use (i.e., on which to send access probes) by applying the same or a similar hash function. In this manner, the available paging channels and access channels can be distributed substantially evenly among the mobile stations in the sector.
In a given sector, the slotted aloha process proceeds according to operational parameters that the base station broadcasts in an “access parameters message” on the paging channel to mobile stations operating in the sector. Under CDMA, for instance, the operational parameters include a specification of the number of access channels, an indication of the power at which a mobile station should transmit its initial access probe in the slotted aloha sequence, the extent to which the mobile station should increase transmit power for each successive access probe, and the number of access probes per slotted aloha sequence, among others.
When the base station receives an access probe from a mobile station, the base station then passes the access probe along to the switch (mobile switching center (MSC)) or other entity, which then responsively sends a registration notification message to the mobile station's home location register (HLR). The HLR then updates the mobile station's profile to indicate where the mobile station is operating (e.g., which switch is serving the mobile station) and may further carry out an authentication process, and then sends a registration response, which propagates to the mobile station.
Various trigger events can cause mobile stations to register with the system. Under CDMA, for instance, a mobile station will generally register (i) whenever it enters a new zone, as indicated by a distinct zone parameter the mobile station receives in an air interface control channel message from the base station, (ii) on a periodic basis, (iii) when the mobile station places a call, as a prerequisite to call placement, and (iv) when the mobile station responds to a page message indicative of an incoming call.
In some situations, the air interface (e.g., a particular sector defined by a base station) can become overwhelmed with too much use. This can happen, by way of example, if too many mobile station registrations occur at once. In a CDMA system, for instance, if access probes from two or more mobile stations line up (by chance) in the same timeslot of an access channel, an “access probe collision” occurs. The result of such a collision is that none of the probes will succeed, principally because the base station will not receive any of the probes in a comprehensible form due to interference between the probes. Thus, each mobile station would have to re-send its access probe, because it would not receive an acknowledgement from the base station.
In many situations, access probe collisions are not very likely to occur, because sufficient timeslots exist on the access channel. However, in situations where many users are turning on their mobile phones or placing calls at once, the number of access probes and access probe collisions can increase exponentially (or at least greatly). For example, after a football game or in an emergency situation, many people within a given sector may use their mobile phones to place calls (e.g., to call 911, to call friends and family, to check voice mail, or for other purposes). Each time a mobile station goes to place a call, as noted above, the mobile station would send an access probe. Consequently, in a situation where many people within a given sector place calls at once, many access probes will be sent at once. In turn, access probe collisions would occur, and so still more (re-try) access probes will be sent. Further, as this is occurring, mobile stations will be periodically registering with the system, which will still further increase the frequency of access probe collisions.
Unfortunately, as an access channel becomes more and more occupied with access probes, two undesired effects will tend to occur. First, the number of access probe collisions will tend to increase, which means that registrations will take longer to successfully complete. In placing and receiving calls, this longer registration process translates into longer call setup time, which in turn translates into an unacceptable user experience. Second, as mobile stations exhaust the slotted aloha process, the number of ultimate access failures will tend to increase. In placing and receiving calls, these access failures will be perceived as blocked calls, which will also result in unacceptable user experience. Therefore, an improvement is desired.